1. Field of the Invention
This invention relates to processes for modifying elastomers, the modified elastomers made thereby, and processes for making products from the modified elastomers.
2. Background Information
The term "elastomer" was first defined in 1940 to mean synthetic thermosetting high polymers having properties similar to those of vulcanized natural rubber, e.g. having the ability to be stretched to at least twice their original length and to retract very rapidly to approximately their original length when released. Representative of these "high polymers" were styrene-butadiene copolymer, polychloroprene, nitrite butyl rubber and ethylene-propylene polymers (aka EP and EPDM elastomers). The term "elastomer" was later extended to include uncrosslinked thermoplastic polyolefins, i.e. TPOs.
ASTM D 1566 defines various physical properties of elastomers, and the test methods for measuring these properties. U.S. Pat. No. 5,001,205 provides an overview of known elastomers comprising ethylene copolymerized with an .alpha.-olefin. As described therein, commercially viable elastomers have various minimum properties, e.g. a Mooney viscosity no less than 10, a weight average molecular weight (M.sub.w) no less than 110,000, a glass transition temperature below -20.degree. C., and a degree of crystallinity no greater than 25%.
A dilemma faced in the production of commercially viable cured elastomers is that a high weight average molecular weight is generally desired to improve physical properties such as tensile strength, toughness, compression set, etc., in the cured product, but the uncured high molecular weight elastomers are more difficult to process than their lower molecular weight counterparts. In particular, the uncured higher molecular weight uncured elastomers are typically more difficult it is to isolate from solvents and residual monomer following polymerization of the elastomer. The uncured higher molecular weight elastomers are also typically more difficult to extrude at high rates, since they are generally prone to shear fracture at lower extrusion rates and require more power consumption by polymer processing equipment such as batch mixers, continuous mixers, extruders, etc., and cause increased wear on the parts of such equipment exposed to high shear stresses, such as expensive extruder components. These disadvantages reduce production rates and/or increase the cost of production.
A conventional approach for resolving this dilemma is to make a relatively low molecular weight elastomer and then fully crosslink the final product to obtain the desired tensile strength, toughness, compression set, etc. A disadvantage of that approach is that the low molecular weight of the elastomer also generally corresponds to a low "green strength" (i.e., strength prior to crosslinking). That disadvantage is particularly noticeable in applications such as coating wire and cable, continuous extrusion of gaskets, etc., where low green strength results in sags or uneven polymer thickness. The present invention addresses these and other disadvantages.